The present invention relates to a display device and an active matrix substrate used for the display device.
Among thin, low-power liquid crystal display (LCD) devices, active matrix LCD devices having switching elements for respective pixels exhibit high-contrast display and fast response speed. Having such high performance, the active matrix LCD devices have found applications in various fields including displays of personal computers and the like and portable TVs, and thus recently the market scale thereof has rapidly expanded. Among others, those having thin film transistors (TFTs) as the switching elements (hereinafter, such display devices are occasionally called “TFT type LCD devices”) have been widely used.
Note that one of the substrates constituting an active matrix LCD device that includes switching elements is herein called an “active matrix substrate”, and in particular, an active matrix substrate including TFTs as the switching elements is called a “TFT substrate”. Typically, an active matrix LCD device includes an active matrix substrate, a counter substrate and a liquid crystal layer interposed between these substrates. A voltage is applied across the liquid crystal layer via a pixel electrode formed on the active matrix substrate and a counter electrode (common electrode) formed on the counter substrate, to change the aligned state of the liquid crystal layer. With this change of the aligned state, the polarized state of light passing through the liquid crystal layer is controlled, to thereby provide display. Note that in an in-plane switching (IPS) mode LCD device, the active matrix substrate includes both the pixel electrodes (display signal electrodes) and the counter electrode.
Improvement in light resistance, together with enhancement in luminance (aperture ratio) and definition, are problems to be overcome in technical development of the TFT type LCD devices. The reason is that intense light incident on a semiconductor layer (silicon layer) constituting a TFT will cause generation of leak current, and this will degrade the display quality. In particular, in a projection display device, among the TFT type LCD devices, higher luminance and higher definition of a liquid crystal panel are strongly demanded. In addition, since the liquid crystal panel is irradiated with more intense light than a liquid crystal panel of a direct-view display device, there is a tight request for the light resistance.
Japanese Laid-Open Patent Publication No. 2001-66638 (Literature 1), for example, discloses a TFT type LCD device that succeeded in increasing the pixel aperture ratio and improving the light shielding property for a semiconductor layer of each TFT. In a TFT substrate disclosed in Literature 1, a storage capacitor is formed in a TFT portion (portion of the TFT substrate in which each TFT is formed) at a position underlying the semiconductor layer of each TFT, and electrodes of the storage capacitor are used as a light-shielding layer.
However, the LCD device disclosed in Literature 1 has a problem that while light incident from below the TFT (from the substrate side) can be sufficiently blocked with the electrodes of the storage capacitor, light incident from above the TFT or the sides thereof fails to be sufficiently blocked.
The reason is as follows. Light incident from above and the sides is supposed to be blocked by a source electrode layer (including a source electrode, a source interconnection and a drain electrode) placed above a gate electrode via an interlayer insulating film and an upper light-shielding layer placed above the source electrode layer, as shown in FIG. 1 of Literature 1. The source interconnection may form parasitic capacitance with another conductive layer, and this will adversely affect a display signal (display signal voltage) passing through the source interconnection. In particular, if the source interconnection forms large parasitic capacitance with a gate interconnection, a phenomenon called crosstalk and a ghost occurs in the LCD device, and this may possibly degrade the display quality. For this reason, in adoption of the LCD device described in Literature 1, it is required to sufficiently thicken an interlayer insulating film formed between the source electrode layer and a gate electrode layer. This increases the gap between the source electrode layer and the semiconductor layer and, as a result, allows light to enter the semiconductor layer through the gap
Literature 1 also discloses a configuration in which a drawing electrode for connecting one of the electrodes of the storage capacitor formed in the lowermost layer and a drain region of the semiconductor layer is formed from a common conductive layer with the gate electrode. In this configuration, since the drawing electrode must be separated from the gate electrode, light is allowed to enter the semiconductor layer through the gap between the gate electrode and the drawing electrode.
Liquid crystal display devices were used to describe the problems related to the conventional active matrix display devices. However, the problems described above are not limited to liquid crystal display devices, but also arise in other non-self emitting display devices including an electrophoresis display device, for example.
In view of the problems described above, an object of the present invention is providing an active matrix display device having a storage capacitor for each pixel, in which light is effectively blocked from entering a semiconductor layer while reduction in aperture ratio is suppressed, and an active matrix substrate used for the active matrix display device.